1. Field of the Invention
This invention generally relates to a method and an apparatus for measuring the height of rotary heads mounted in a magnetic recording & reproduction system.
2. Prior Art
There is known a method of measuring the height of a rotary head mounted or equipped on a rotary head unit in a magnetic recording & reproduction system without removing the rotary head unit itself from the magnetic recording & reproduction system. Recently, size of such a measuring system and manufacturing costs for it have been steadily reduced. For example, Unexamined Japanese Patent Application No. HEI 5-120650, published in 1993, discloses a system capable of measuring the height of a rotary head using rotary head output signal obtained in the still reproduction mode of the magnetic recording & reproduction system, as shown in FIGS. 9 through 12.
FIG. 9 is a view showing an arrangement of a rotary head device where a plurality of rotary heads 92-95 are provided an a rotary drum 91 equipped in a magnetic recording & reproduction system 100. As shown in FIG. 10, the magnetic recording & reproduction system 100 is associated with a conventional rotary head height measuring system 200 which comprises a detecting circuit 201, an A/D converter 202 and a computer 203. Rotary heads 92 and 94 belong to a second channel (i.e. CH2) having the same azimuth angle, while rotary heads 93 and 95 belong to a first channel (i.e. CH1) having the same azimuth angle but opposite to the azimuth angle of the rotary heads 92 and 94.
Meanwhile, rotary heads 92 and 93 are a couple of rotary heads belonging to channel B (i.e. CHB) and rotary heads 94 and 95 are another couple of rotary heads belonging to channel A. (CHA). Channel A and channel B are alternately selected or switched in response to a head switching signal.
An operation of the above-described conventional rotary head height measuring system will be explained hereinafter with reference to FIGS. 11 and 12. FIG. 11 is a view illustrating the locus or trajectory of rotary heads on a tape, used for explaining the operation of the conventional rotary head height measuring system 200. As shown in FIG. 11, numerous oblique tracks 72-76 are recorded consecutively on the surface of a tape 71.
A track, denoted by reference numeral 72, is recorded with the azimuth angle corresponding to the CH2 heads (i.e. rotary heads 92 and 94. A track, denoted by reference numeral 73, is recorded with the azimuth angle corresponding to the CH1 heads (i.e. rotary heads 93 and 95. Each track is administrated to have a predetermined width.
Reference numeral 74 represents the locus of rotary CH2 heads (i.e. rotary heads 92 and 94 in the still reproduction mode when these rotary heads 92 and 94 are positioned correctly at their predetermined regular height. Reference numeral 75 represents the locus of CH1 heads (i.e. rotary heads 93 and 95 in the still reproduction mode when these rotary heads 93 and 95 are positioned correctly at their predetermined regular height.
Meanwhile, reference numeral 76 represents the locus of rotary head 94 in the still mode when this rotary head 94, one of CH2 heads, is positioned lower than the predetermined regular height by an amount of .DELTA.h3.
FIG. 12 is a time chart showing respective outputs of rotary heads 92-95 in the still reproduction mode shown in FIG. 11. Reference numeral 81 represents the head switching signal whose output alternates at the same intervals between high (CHA) and low (CHB). This interval is referred to as head switching duration, hereinafter. Reference numeral 82 represents the output waveform of CH1 heads (i.e. rotary heads 93 and 95) obtained when these rotary heads 93 and 95 are precisely positioned at their predetermined regular height.
Reference numeral 83 represents the output waveform of CH2 heads (rotary heads 92 and 94) obtained when these rotary heads 92 and 94 are precisely positioned at their predetermined regular height. Reference numeral 84 represents the output waveform of the rotary head 94 obtained when this rotary head 94, one of the CH2 heads, is dislocated lower than the predetermined regular height by the amount of .DELTA.h3.
When the rotary head 94 (one of CH2 heads) is positioned lower than the predetermined regular height by the amount of .DELTA.h3, its locus 76 is deviated from the correct locus 74 as shown in FIG. 11. In such a case, the output peak of rotary head 64 is offset from its regular position by an amount of time difference .DELTA.t5 as shown in FIG. 12. Hence, the time difference between concerned two peaks of channel CH2 is differentiated.
T3'&gt;T4' PA1 .DELTA.h represents the altitudinal dislocation of the concerned rotary head; PA1 P represents the number of peaks appearing on the output signal of each rotary head within each switching interval; PA1 A represents the number of kinds of azimuth angles of the rotary heads; PA1 Tw represents the track width; PA1 .DELTA.t represents the peak time difference; and PA1 T represents each switching interval.
The time difference .DELTA.t5 is expressed by the following equation. EQU .DELTA.t5=(T3'-T4')/2
In this conventional system, only one output peak appears on the rotary head signal within each head switching duration. The rotary heads and tracks recorded on the tape respectively correspond to two kinds of azimuth angles. Hence, a shift amount of the concerned output peak during a period of time from a building up of each head switching duration to its trailing edge corresponds to a change of the height of the rotary head, mounting two times the track width, as defined by the following equation. EQU .DELTA.h3/.DELTA.t5=2.times.Tw/T
where Tw represents the track width, while T represents the time interval of each head switching duration; T=(T1+T2)/2.
Accordingly, the altitudinal dislocation .DELTA.h3 is obtained from the above-defined relationship. EQU .DELTA.h3=2.times.Tw.times..DELTA.t5/T
As shown in FIG. 10, magnetic recording & reproduction system 100, when it operates in the still reproduction mode, sends the rotary head output signal to the rotary head height measuring system 200. The detecting circuit 201 and A/D converter 202 cooperatively detect each peak position of the rotary head output signal, and the computer 203 calculates the time difference .DELTA.t5 and obtains the altitudinal dislocation .DELTA.h3 referring to the above-described equation.
However, according to the above-described conventional rotary head height measuring system, there were problems derived from the nature of the still reproduction where the tape travelling speed was zero. In the magnetic recording & reproduction system, appropriate tension is normally applied on the tape when it travels, providing uniformity in the contact between the rotary head and the tape and assuring adequacy in the rotary head output signal.
On the contrary, when the tape is stopped in the still mode, tape tension cannot be stabilized and also tape restriction cannot be steady in the transverse direction of the tape travelling in the longitudinal direction thereof. As a result, contact between the rotary head and the tape is not uniform. As shown in FIG. 12, swelling or distortion 85 appears on the waveform of the rotary head output signal. Accordingly, it was not possible to obtain a correct peak time difference, resulting in the deterioration of measuring accuracy in the measurement of the rotary head height.
Furthermore, when the width of the rotary head is different from the width of the track recorded on the tape, the rotary head output signal may cause a flattened peak in its waveform, which will make it possible to detect the peak time difference accurately.
Still further, if a method of identifying the maximum output value as corresponding to a peak position is adopted in the case where the width of the rotary head is different from the width of the track recorded on the tape, it will encounter the problem that the maximum output value may be detected in a relatively wide zone in the vicinity of the actual peak position due to the flattened peak in the waveform, making it difficult to accurately find out the actual peak position.